1. Field of the Invention
The present invention relates to a liquid discharge head having a nozzle for discharging liquid, and a head cartridge and an image forming apparatus that use such a liquid discharge head.
2. Description of the Related Art
In recent years, owing to the spread of the Internet, digital cameras, and so on, the demand for high-gradation color printing has been increased and, following it, ink jet printers have been improved to have higher performances. As means for obtaining a high-quality print image with high fineness and high gradation, those methods such as (1) and (2) below are considered to be particularly effective.
(1) A volume of ink to be discharged is reduced and a nozzle array pitch is narrowed, thereby to improve the resolution.
(2) With respect to particular color ink, a plurality of nozzle arrays are prepared for discharging, respectively, a plurality of (at least two) color inks that differ in ratio of a contained color material, i.e. in concentration of the color material, and the high-concentration ink and the low-concentration ink are selectively printed in an overlapping manner as required, thereby to improve gradation.
However, for obtaining the high-quality print image with high fineness and high gradation by reducing as much as possible a volume of each ink droplet to be discharged so as to realize the method (1), it is necessary to hit (place) more ink droplets on a recording medium with high accuracy and record them, and thus, nozzles for discharging ink droplets of the stable volume and hitting them on the recording medium with high accuracy as well as a high frequency response of an ink discharge head are required.
Further, for realizing the method (2), nozzle arrays for discharging high-concentration ink and low-concentration ink, respectively, are necessary with respect to particular color ink, so that a configuration of a print head becomes complicated. For discharging such small ink droplets from the nozzles, there has been proposed a print head of the type that causes bubbles growing due to film boiling following heating of ink to communicate with the air via nozzles, as disclosed in, for example, JP-A-04-10940, JP-A-04-10941, and JP-A-04-10742. For distinguishing it from the old bubble jet type that discharges ink droplets without causing bubbles growing due to film boiling to communicate with the air, it is sometimes called the bubble through type.
In the print head of the old bubble jet type that discharges ink droplets without causing bubbles growing due to film boiling to communicate with the air, as the size of an ink droplet discharged from a nozzle is reduced, it is necessary to reduce a sectional area of an ink flow passage communicating with the nozzle, so that there arises a disadvantage that the discharge efficiency is lowered to drop the discharge speed of the ink droplet discharged from the nozzle. If the discharge speed of the ink droplet is lowered, it is possible that a discharge direction thereof becomes unstable, and further, viscosity of the ink is increased following evaporation of moisture while the print head is stopped in operation, and thus the discharge state becomes further unstable to cause initial discharge failure and so on, thus leading to lowering of the reliability.
In contrast, in the print head of the bubble through type in which bubbles communicate with the air, the size of an ink droplet can be determined only from a geometrical shape of a nozzle. Therefore, there are advantages that it is suitable for discharging small ink droplets, it tends to be free of an influence such as temperatures, and the discharge amount of each ink droplet is very stable as compared with the print head of the old bubble jet type. Thus, it is possible to obtain the high-quality print image with high fineness and high gradation relatively easily.
For obtaining the high-quality print image with high fineness and high gradation, a print head configuration in combination of the foregoing methods (1) and (2) is considered to be particularly effective. For obtaining a print image at high speed, it is also effective to shorten a period for discharging ink, or form each recording pixel with ink of a large discharge volume, thereby suppressing the recording density. Therefore, if a discharge volume of low-concentration ink is set to be larger than a discharge volume of high-concentration ink with respect to particular color ink, and an image is recorded on a recording medium by combination of ink droplets of them, the total number of times of discharging ink and a period therefor can be suppressed, so that there can be realized an ink droplet discharge head that is energy saving and highly accurate. Accordingly, the high-quality print image with high fineness and high gradation can be obtained at high speed.
When carrying out printing by discharging an ink droplet of an extremely small amount from one nozzle, the bubble through type is particularly suitable in the ink jet printer as described above.
FIG. 9 shows a discharging state of ink droplets when the ink droplets are continuously discharged from all the nozzles while scan-moving the print head of the ink jet type together with a carriage along a print medium at high speed, thereby performing so-called solid printing relative to the print medium. The scan-moving direction of a print head 101 is perpendicular to the sheet of FIG. 9, and nozzles (not shown) are arrayed right and left in the figure. When image data is solid, all the discharge energy generating portions (not shown) corresponding to the respective nozzles are driven at high driving frequencies. Therefore, following motions of ink droplets 103 discharged from the nozzles toward a print medium 102, the ambient air having viscosity also moves induced by the motions of the ink droplets 103. As a result, the vicinity of a nozzle surface 104 where the nozzles of the print head 101 open tends to be reduced in pressure as compared with portions around the print head 101, so that the ambient air flows into the pressure-reduced region as air flows. It has been confirmed that, due to influence of the air flows, the ink droplets 103 discharged from the nozzles, particularly those nozzles located on both end sides in a nozzle array direction, are drawn toward the center in the nozzle array direction, so that the ink droplets 103 are not discharged to expected positions relative to the print medium 102. Accordingly, a plurality of discharged liquid droplets are drawn toward the center.
FIG. 10 exemplarily shows an image of solid printing that is formed on the print medium when the solid printing is carried out by a plurality of times of scanning movement of a carriage under such a phenomenon. The carriage is scan-moved along with the print head perpendicularly to the sheet of the figure. It is seen that a white stripe 107 is formed between a solid image 105 formed by the previous scanning movement and a solid image 106 formed by the subsequent scanning movement.
It has been found through study of the inventors that such a disadvantage occurs particularly notably in the ink jet printer of the bubble through type wherein the nozzle array pitch is set narrow and ink droplets each of a small amount of 10 pl or less can be discharged at a short period through one driving operation, and the degree of the disadvantage differs depending on a difference in volume of an ink droplet discharged. Table 1 shows data about an end mis-alignment amount (half value of white stripe 107) when the nozzle array pitch is 1200 dpi (21 xcexcm).
The reason therefor is that since the ink droplets are continuously discharged from all the nozzles while scan-moving the print head together with the carriage along the print medium at high speed, various factors such as a shape and a scanning speed of the carriage, a position of the nozzle array, a distance between the nozzle and the print medium, and a size and a discharge speed of an ink droplet, are complexly entangled. Among these factors, what the present invention aims to solve is to prevent the degree of the end mis-alignment (disadvantage that a plurality of discharged liquid droplets at end portions are drawn toward the center) from differing depending on the size of ink droplets. Another is to perform the optimum correction of end mis-alignment amounts that influence each other due to coexistence of ink droplets having different sizes. For example, when a nozzle array discharging about 4 pl from each nozzle and a nozzle array discharging about 2 pl from each nozzle coexist and a carriage scans at high speed while causing both nozzle arrays to discharge simultaneously, ink droplets each of about 2 pl located on the downstream side of the nozzle array discharging ink droplets each of about 4 pl exhibit an end mis-alignment amount that is 1.5 to 3 times an end mis-alignment amount when discharged alone. Conventionally, it was possible to ease the foregoing disadvantage by suppressing the driving frequencies for the discharge energy generating portions, or by reducing the number of those portions to be driven while the carriage performs scanning once. However, if the driving frequencies for the discharge energy generating portions are lowered or the number of the driven portions is set small, the printing speed is lowered so that the users"" demand for high-speed printing can not be satisfied.
Therefore, an object of the present invention is to provide a liquid discharge head that, even in an ink jet printer in which nozzle arrays having different ink discharge volumes coexist so that ink droplets having different sizes are discharged simultaneously, aims to prevent deviation of ink droplets discharged from nozzles located on both end sides in a nozzle array direction of each nozzle array, thereby to prevent occurrence of a white stripe upon solid printing, and further provide a head cartridge and an image forming apparatus using such a liquid discharge head.
For accomplishing the foregoing object, according to the present invention, there is provided a liquid discharge head having a plurality of nozzle arrays each provided with a plurality of nozzles and each arranged substantially in parallel to a print medium conveying direction, and a plurality of discharge energy generating portions for discharging liquid from the nozzles, respectively, the liquid discharge head moved to scan in a direction crossing the conveying direction, the liquid discharge head, wherein each of the nozzle arrays has a first nozzle group in which a pitch of the nozzles is set to a first pitch, and second nozzle groups arranged on both end sides of the corresponding nozzle array and each having at least a third nozzle group provided with the nozzles arranged at a second pitch greater than the first pitch, and the first pitch and the second pitch are set so as to differ depending on a volume of a liquid droplet to be discharged.
As described above, the nozzle array has the first nozzle group, and the second nozzle groups each having the third nozzle group, and the second pitch of the nozzles on both end sides of this nozzle array is set greater than the first pitch. With this configuration, such a state can be prevented individually per discharge volume that the vicinity of the nozzles tends to be reduced in pressure as compared with portions around the liquid discharge head, so that the ambient air flows into the pressure-reduced region as air flows and, due to influence of the air flows, the liquid discharged from the nozzles, particularly those nozzles located on both end sides in a nozzle array direction, are drawn toward the center in the nozzle array direction, hence, the liquid is not discharged to expected positions relative to the print medium.
Further, the pitches of the nozzles are configured to differ depending on the volume of the liquid droplet to be discharged. With this configuration, even if there coexist two or more kinds of volumes of liquid droplets to be discharged simultaneously, the respective nozzle pitches can be corrected by proper amounts.
It may be configured that a volume of a liquid droplet discharged from each of the nozzles arranged at the first pitch is smaller than a volume of a liquid droplet discharged from each of the nozzles arranged at the second pitch.
Further, it may be configured that each of the second nozzle groups includes only the third nozzle group that includes the nozzle located at an end of the nozzle array.
Further, it may be configured that each of the second nozzle groups includes a fourth nozzle group provided with the nozzles arranged at the first pitch and including the nozzle located at an end of the nozzle array.
Further, it may be configured that liquid to be discharged is recording ink selected from the group consisting of same-color ink, thick-color ink and light-color ink, and/or treatment liquid for adjusting a printing property of ink relative to a print medium.
Further, it may be configured that liquid to be discharged is recording ink, ink droplets to be discharged have two different volumes, a volume ratio of the two difference volumes is 1.5 to 3 times, an ink droplet having the larger volume has a lighter color, and an ink droplet having the larger volume is discharged from each of the nozzles set to the second pitch. Alternatively, the liquid to be discharged may be recording ink of the same color.
Further, it may be configured that each of the discharge energy generating portions has an electro-thermal converter that generates thermal energy for causing film boiling in liquid to discharge the liquid from the nozzles.
According to the present invention, there is provided another liquid discharge head having a plurality of nozzle arrays each provided with a plurality of nozzles and each arranged substantially in parallel to a print medium conveying direction, and a plurality of discharge energy generating portions for discharging liquid from the nozzles, respectively, the liquid discharge head moved to scan in a direction crossing the conveying direction, the liquid discharge head, wherein a volume of an ink droplet to be discharged from each of the nozzles differs per the nozzle array, a pitch of the nozzles arranged on both end sides of each of the nozzle arrays is greater than a pitch of the nozzles arranged at a center portion of the corresponding nozzle array, and the pitch of the nozzles arranged on both end sides of the nozzle array where a volume of an ink droplet is large, is greater than the pitch of the nozzles arranged on both end sides of the nozzle array where a volume of an ink droplet is small.
According to the present invention, a head cartridge is characterized by comprising the liquid discharge head of the present invention and a liquid tank storing liquid to be supplied to the liquid discharge head.
It may be configured that the liquid tank is detachable relative to the liquid discharge head via attaching/detaching means.
According to the present invention, an image forming apparatus is characterized by comprising a mounting portion for the liquid discharge head of the present invention, wherein an image is formed on a print medium using liquid discharged from nozzles of the liquid discharge head.
As described above, according to the image forming apparatus of the present invention, since an image is formed on the print medium by the liquid discharge head of the present invention, such a state can be prevented individually per discharge volume that the vicinity of the nozzles tends to be reduced in pressure as compared with portions around the liquid discharge head, so that the ambient air flows into the pressure-reduced region as air flows and, due to influence of the air flows, the liquid discharged from the nozzles, particularly those nozzles located on both end sides in a nozzle array direction, are drawn toward the center in the nozzle array direction, hence, the liquid is not discharged to expected positions relative to the print medium. Therefore, even if there coexist two or more kinds of volumes of liquid droplets that are simultaneously discharged, since respective nozzle pitches are corrected by proper amounts, a high-quality print image with high fineness and high gradation, which is free of occurrence of a white stripe even if solid printing is carried out, can be obtained.
It may be configured that the mounting portion has a carriage that is movable for scanning in a direction crossing a print medium conveying direction.
Further, it may be configured that the liquid discharge head is detachably mounted on the carriage via attaching/detaching means.